This invention relates generally to switch-mode power converters. More particularly, this invention relates to methods and apparatus for digital peak current mode control for switch-mode power converters.
Switch-mode power converters typically include one or more semiconductor switches and energy storage elements, such as inductors and capacitors, and operate by switching the energy storage elements between various circuit configurations at a predetermined switching frequency. In a pulse-width modulated (“PWM”) converter, the output voltage or current of the power converter can be regulated by varying the duty cycle of the control signal that is applied to the switches.
Analog control methods traditionally have been used to provide line and load regulation of switch-mode power converters, such as DC-DC, AC-DC, DC-AC and AC-AC converters. Analog control techniques offer simplicity, high bandwidth and low implementation cost. Conventional analog control techniques for switch-mode power converters include voltage-mode and current-mode control.
Voltage-mode control is a single-loop control technique that causes the converter output voltage to track a reference voltage. In particular, the output voltage is compared to the reference voltage, and the error signal is used to set a switch duty ratio of the converter. By varying the switch duty ratio, the average voltage across the inductor, and hence the inductor current, are adjusted. This causes the output voltage to follow the reference voltage.
Current-mode control, in contrast, is a two-loop control method that includes current and voltage control loops, and causes the inductor current to track a reference current. In the voltage control loop, the error signal between the output voltage and reference voltage is used to generate a reference current. The current loop compares the reference current to the inductor current to control the switch duty ratio. In this way, some aspect (e.g., peak, valley, average, or some other aspect) of the inductor current tracks the reference current, and the output voltage tracks the reference voltage.
Peak current mode control refers to a control mode in which the peak value of the inductor current tracks the reference current, and offers numerous advantages for some power converter applications. Indeed, peak current mode control phase-shifted PWM (“PSPWM”) full bridge is the topology of choice for many power supply manufacturers. In such a control scheme, control signals for switches on one leg of the converter are phase-shifted relative to the control signals for switches on the other leg, with the phase shift determined based on peak inductor current. The benefits of this topology include high efficiency, cycle-by-cycle current limit, transformer flux balance, and near ideal audio susceptibility. Such converters have traditionally been implemented using analog control techniques.
Analog control techniques, however, have several disadvantages, such as large component count, low flexibility, low reliability, and high sensitivity to temperature, ageing and component tolerances. Moreover, previously known peak current mode controllers typically include high speed analog comparators and require numerous external components for configuration.
In recent years, digital control techniques for power converters have become more practical, particularly with the advent of high-performance, high speed, low cost digital signal processors, analog-to-digital (“A/D”) converters and digital-to-analog (“D/A”) converters. Indeed, digital controllers offer many advantages, including programmability, flexibility, simplicity, and immunity to environmental variations.
Although various digital control techniques for power converters have been successfully implemented, a fully digital implementation of peak current mode control has proven difficult to achieve. In particular, previous attempts to implement a digital controller that senses and reliably terminates a peak inductor current without producing limit cycles have been met with limited to no success.
However, the benefit of digital peak current mode control could be used to leverage the advantages of commercially available, low-cost, high-performance digital processors. Such an implementation could result in a net reduction in component count and cost, and also provide numerous other advantages.
Accordingly, improved methods and apparatus for digital peak current mode control for switch-mode power converters are desirable.